1. Field of the Invention
The present invention relates to a glow plug electrification control apparatus for controlling the supply of electric current to a glow plug that assists startup of an internal combustion engine, and to a glow plug electrification control system using the same.
2. Description of the Related Art
In general, a glow plug has a resistance heater (heater section) which generates heat upon supply of electric current thereto. The glow plug is configured such that a resistance heater is attached to a metallic shell, and in use is attached to the engine block of a diesel engine such that the distal end of the resistance heater is located within a combustion chamber.
A glow plug electrification control apparatus has been known as an apparatus for controlling the supply of electric current to such a glow plug. Since such a glow plug has a relatively high resistance, a conventional glow plug electrification control apparatus is configured as follows. When a key switch is turned to an ON position, a switch (switching element) between a battery and a glow plug is maintained ON so as to supply a large current to the glow plug and raise the temperature of the resistance heater to a first target temperature (e.g., 1300° C.) which is sufficiently high for starting the engine. Such a step is generally called a “pre glow” or a “pre glow step.” A glow plug capable of quick heating can raise the temperature of its resistance heater to the first target temperature within a few seconds (see Patent Documents 1 and 2).
In recent years, a glow plug of a quick temperature raising type has emerged which can raise the temperature of its resistance heater to 1000° C. or higher within about 2 seconds, by further reducing the resistance of the heater section.
In a known control method performed during temperature rise of a glow plug, the amount of cumulative power supplied to the glow plug is controlled so as to raise the temperature of the glow plug to a sufficiently high temperature without being affected by the battery voltage and to prevent excessive temperature rise. Specifically, voltage applied during the temperature rise and current flowing through the glow plug during the temperature rise are measured; electric power supplied to the glow plug is calculated and integrated so as to calculate the cumulative amount of electric power; and the temperature of the glow plug is raised until the cumulative amount of electric power reaches a predetermined value (see Patent Document 3).    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. S56-129763    [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. S60-67775    [Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 2004-232907
3. Problem to be Solved by the Invention
However, even glow plugs of the same part number, which are industrially handled as the same part and are regarded as having the same performance, show variation in their respective resistances. Accordingly, when a battery voltage is applied via a switching element to a glow plug having a relatively low resistance, a relatively large current flows therethrough. As a result, the speed of temperature rise is high, the glow plug reaches a high temperature within a short period of time, and the cumulative amount of electric power supplied to the glow plug reaches a predetermined value within a short period of time. In addition, since the temperature of the glow plug is raised within a short period of time, the amount of heat which escapes from the glow plug to an engine head or the like during the temperature rise decreases. Thus, the glow plug having a lower resistance reaches a higher temperature, as compared with a glow plug having a higher resistance, even when the same cumulative amount of electric power is supplied thereto.
Meanwhile, when the battery voltage is applied via the switching element to a glow plug having a relatively high resistance, a relatively small current flows therethrough. As a result, the speed of temperature rise is low, the glow plug requires a long period of time to reach a high temperature, and a long period of time is required for the cumulative amount of electric power supplied to the glow plug to reach the predetermined value. In addition, since the glow plug requires a long period of time to reach a high temperature, a larger amount of heat escapes from the glow plug to the engine head or the like during the temperature rise. Thus, the glow plug having a higher resistance can reach only a lower temperature, as compared with a glow plug having a lower resistance, even when the same cumulative amount of electric power is supplied.
That is, due to variation in resistance among glow plugs, variations arise not only with regard to the temperature rising time, but also the ultimate temperature that the respective glow plugs can reach. Thus, various problems arise, such as variation in engine ignitability.